ABSTRACT: The coral Pocillopora damicornis hosts genetically distinct and novel types of dinoflagellate symbionts at the high-latitude site of Lord Howe Island (LHI), yet why these novel types exist at this marginal site is unknown. In this study, it was determined whether one of the novel Symbiodinium types at LHI is physiologically adapted for this high-latitude site, where water temperatures annually range from 18°C to 26°C. Low and high short-term thermal bleaching thresholds of the coral-symbiont partnership were measured as 14°C and 30°C. Photochemical sensitivity to temperature (15°C and 29°C) and light treatments (100% and 40% of sunlight), measured as effective quantum yield and fast induction curves, were determined over a 72-h period. A greater effect on the photochemical reactions of LHI P. damicornis symbionts was recorded in response to a 3°C temperature increase from annual maxima than a 3°C temperature decrease from annual minima. Corals did not bleach when temperature was reduced to 15°C for 72 h; in contrast, a 92% decline in photochemical efficiency was recorded in the 29°C treatment (ΔF:F'm < 0.05), compared to 35% loss in the control (20°C). For the first time, a Pulse Efficiency Analyser fluorometer was used to assess the effect of reduced temperature on symbionts, showing a reduced rate of QA reduction, further enhanced by high light levels. This type of Symbiodinium at LHI may be specialized for cooler and more variable temperatures, so contributing to the success of corals at this marginal location.

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